RoadVista has a publication "Guide to Retroreflective Measurement Principles for Highway Signs and Markings" at http://www.roadvista.com/guideSummary.shtml which is useful in understanding retro-reflectance of highway signage.

Delta has a variety of retro-reflectometers, both road material measurement units and a gun for highway signs all of which conform to various DIN, ASTM and CEN specifications. Also has a 0/d sphere instrument for daylight visibility (luminous coefficient) of road markings that conforms to EN 1436 Road marking materials - Road marking performance for road users.